Organic fuels can be used to generate electrical power by converting the energy released from the electrochemical reactions of the organic compounds into electrical current. Organic fuels like methanol are renewable and environmentally safe since the typical products from the electrochemical reactions are carbon dioxide and water. The use of fuel cells therefore avoids many of the environmentally detrimental consequences associated with burning fossil fuels, such as air pollution caused by exhaust from gasoline-powered internal combustion engines. The interest in organic fuel cells stems from the promise of organic fuels as an alternative, environmentally friendly energy source to non-renewable fossil fuels.
Direct liquid feed fuel cells use an aqueous solution of an organic fuel such as methanol in water or an acid-base system. The mixture is circulated past the anode of the cell wherein the organic fuel is oxidized, resulting in the production of electrons and the release of protons. The movement of electrons from the anode to the cathode occurs through the electrical load that generates electrical current. The protons generated at the anode traverse the membrane that separates the anode and cathode sections to permit electrochemical reduction of oxygen at the cathode. Electrical power is thereby generated by the simultaneous electrochemical reactions that occur at the anode and cathode of the fuel cell.
In the case of a direct methanol fuel cell (DMFC), methanol (CH3OH) is used as the fuel, which undergoes oxidation to carbon dioxide (CO2) at the anode according to the following electrochemical reaction:CH3OH+H2O→CO2+6H++6e−.
Oxygen undergoes reduction to water at the cathode according to the following electrochemical reaction:O2+4H+4e−→2H2O
The overall electrochemical reaction in the direct methanol fuel cell is:2CH3OH+3O2 →2CO2+4H2O+Electrical Energy
Currently available DMFC systems are expensive and display low efficiency and low power density. A key contributor to the limited utility of DMFC systems is the limited performance exhibited by the state of the art electrocatalysts. Since methanol electrooxidation is a kinetically limited process, high catalyst loading levels are required. Typical catalyst materials are based upon noble metals like platinum (Pt) and ruthenium (Ru). To provide ample current levels from a DMFC system, approximately 2 to 10 milligrams (mg) of noble metal must be loaded onto each square centimeter of catalyst surface, which translates into a catalyst cost of up to approximately $3,000 per kilowatt of power generated from the DMFC system. The widespread commercial feasibility of DMFC systems requires the catalyst cost be approximately $100 per kilowatt, which means that catalyst loading must be reduced to 0.5 mg of noble metal per square centimeter of catalyst surface.
Furthermore, the fuel-to-electric efficiency of state of the art DMFC systems is about 22% and the power density of practical systems is about 15 W/kg. The feasibility of extending DMFC systems into portable applications requires smaller, more efficient systems, which means that the power density at the stack level should be doubled and that the overall efficiency should be increased to at least 35-40%. Notwithstanding the current challenges confronting DMFC system development, interest in DMFC systems is fueled by their potential to offer several times the energy storage capacity of advanced rechargeable lithium batteries. Thus, there is a significant need to develop electrocatalysts that display improved catalytic activities for this potential to become realized.
The prior art attempts to improve catalytic activities have focused on the development of the optimal surface area-to-volume ratio of the catalyst materials as a means for improving catalytic activity and for reducing the content of noble metal loading. Previous efforts devoted to improve anode catalysts have focused on using non-noble metal additives such as Nickel (Ni), Cobalt (Co), Vanadium (V), Iron (Fe), Copper (Cu), and Molybdenum (Mo) to enhance catalytic activity. The prior art compositions displayed lower electrode current densities than those found for commercial Platinum/Ruthenium (Pt/Ru) powders. These non-noble metal containing catalysts were not substantially resistant to the corrosive acidic environment in polymer electrolyte membrane fuel cells. Corrosion of cell components by the acid/alkali electrolyte imposes significant constraints on the materials that can be used for the cell. The metal compositions selected for the electrocatalyst must display robust resistance to the corrosive environments of the fuel cell environment.
The invention disclosed herein addresses the feasibility of improving the cost effectiveness of electrocatalysts by reducing Platinum (Pt) content of Pt/Ru catalytic powders through substitution of a portion of Pt with non-noble metals. The invention is directed to robust combinatorial fabrication methods for producing novel metallic material compositions with low Platinum (Pt) content that display substantial resistance to corrosive acids and at least the catalytic activity on a per mole Pt basis observed for prior art Pt/Ru binary alloys. As used herein, an alloy is composed of a mixture of two or more metals. The invention thereby provides a significant advance in the state of the art for material compositions that have applications in DMFC anode catalyst films and powders as well as in other applications where it is desirable to utilize metallic material compositions with reduced Pt content.